Limit switches, inertia and controllers

[Neale]

Sensing distance is one thing, but the other thing to watch is the hysteresis - difference between switch-on and switch-off points. I had problems with my machine where I homed Z, then X and Y together. However, because of the tiny difference between on/off switch points, the vibration during X/Y homing was enough to trigger the Z home switch which by this time had become a limit switch. Fortunately, there is a Mach3 setting which offsets the home position slightly after homing which avoided the problem, but it's something to watch for. Spec sheets for more expensive switches seem to suggest something like 10% of the switching distance but mine was rather less than that.

[routercnc]

On this general subject there are at least 4 ‘end of axis’ type features:
Home (physical sensor to home and zero the axis)
Soft limits (software detection for end of travel)
Limits (physical sensor for end of travel and invoke stop or estop depending on your setup)
Buffer (mechanical buffer for physical hardstop- not often seen on DIY machines but VMCs often have a rubber stop on the ballscrew as a last resort as these machines are heavy and move fast!)

I mention this as the other day I was jogging my machine around and managed to almost hit the home switch ( proximity type). I don’t have soft limits on and the home switch also feeds into my end of travel pin and does not invoke an estop (by design). I haven’t had a chance to investigate further but it looked like when I approached the home at full jog speed it treated it like I had released the jog key and decelerated to a gentle stop. But because it was moving quickly, unlike during homing, it used up all of the 4mm travel. If this is the case I will have to tune the accel / decel to be quicker or set up soft limits. My sensors are head on as I couldn’t get them side on. Anyone else seen this or do you estop if the end of travel is detected?

[Clive S]

I don’t have soft limits on

Is there a reason not to use the soft limit as that would stop it

[hanermo2]

Fwiw..
On steppers, the max acceleration with 48V DC, 542 drivers, Nema 23 3Nm steppers, 3:1 via HTD 5/15 mm, about 0.8 secs to max speed.
My table on a moving table VMC is over 200 kg in mass.
It stops in maybe 1- 2 mm from max speed. Using hw pulsing from a good generator, lately pokeys.

On 400W ac servos, Nema 23, 3.9 Nm peak, it stops in about 1 mm or less.

On the lathe, the new saddle assy == 150 kg.
32 / 4 mm ballscrew, 750W cont ac servos, 10 Nm peak.

X starts and stops in much less than 1 mm, if I was to use high acceleration - I don´t.
I use about 20% of max acc. for "normal" use.

Steppers in the past used to do similar, with much lower top speeds, and much less accuracy.

My c axis spindle and z axis (lathe) have about 10 kW of power in terms of inertia (I know its not kW technically).
Ie industrial turning centers (HAAS ST10) of 10 kWh cont. has about the same peak torque (102 Nm at 1200 rpm) as I have (90 Nm) 0-1000 rpm.
Using a 10" 4-jaw chuck.

The steppers at 1:3 ran about 600-700 rpm, 200 rpm+ at screw of 5 mm rise.

The servos run 3000 rpm, but I only use one third or 1000 rpm.
Very fast accelerations are like hitting the screw mount bearings with a big hammer, 10 kg, literally.
It wears the bearings out fast.

My mounts could take 3000 kgf loads, you could stand a truck on them.

Recap:
Lathes and mills typically go to max-speed in about 1 mm - 2 mm of travel.
Steppers are ok for full-out, and servo systems depend on a lot of stuff.

Anecdote:
The ac servos go to 3000 rpm in about 20 ms, 0.02 secs. Both 60V 400W and 220V 750 W servos.
It is likely the lathe servos could do full-peak accelerations, with the saddle, to 3000 rpm, at 0.02 secs or so.

At 10 Nm direct drive peak, the push force is about 650-700 kgf, 7000 N, near 1900 lbs of force.
For 150 kg mass on saddle assy, ==> 4.3 kgf.
43 m s/s acceleration.

At 1000 rpm x 5 mm, = 5 m/min. /60 = .083 m/sec.
8.3 cm/sec.
In 0.02 secs, the travel is 0.166 cm, = 1.6 mm.

[routercnc]

[QUOTE=Clive S;107130]Is there a reason not to use the soft limit as that would stop it[/QUOTE
No reason. Just need to get a session back in the workshop again !

[Clive S]

[QUOTE=routercnc;107141]

Is there a reason not to use the soft limit as that would stop it[/QUOTE
No reason. Just need to get a session back in the workshop again !

I know how you feel

[Neale]

Main reason for not using soft limits seems to be that you can't jog with soft limits on until you have homed the machine. I don't find that a limitation, but it all depends on things like where you home the machine, where you park the spindle when loading/unloading work, work area and rapids speed.

I've been using my machine this evening, but the cold drove me indoors eventually.

[Voicecoil]

Aaaah, someone else who has a cold workshop - welcome to the club